What is the meaning of 'Postmodernism'?

Postmodernism is the sillier sort of art jargon. Modern means "in the current fashion" but currency is not static; yesterday's modern is today's old hat, and so-called "modern art" has become very old-fashioned. We should logically refer to whatever we are doing now as "modern," but because "modern art" refers specifically to the first half of the 20th Century, we have to use "postmodern" to describe the art of today.